In “The Synthesis of Bisitaconamic Acids and Isomeric Bisimide Monomers,” Galanti, A. V. et al., Journ. Poly. Sci.: Polymer Chemistry Edition, Vol. 20, pp. 233-239 (1982) a method is disclosed for the preparation of biscitraconimides in the form of an isomeric mixture of the citraconic and itaconic imides.
In “The Development of Tough Bismaleimide Resins,” Stenzenberger, H. D., et al., 31st International SAMPE Symposium, Vol 31, pp. 920-932 (1986) it is disclosed that bismaleimides are prime candidates for carbon fiber reinforced composites because of their properties. However, the article also points out that these materials tend to be brittle. Thus, several attempts have been made to improve the fracture toughness of the polymerized bismaleimides.
In “Bismaleimide Resins the Properties and Processing of ‘Compimide’ BMI Resins,” Segal, C. L., et al., 17th Nat. SAMPE Conference 17, pp. 147-160 (1985) formulated bismaleimides are cured with the anionic catalysts DABCO and 2-methylimidazole. It was concluded that, in general, the curing of formulated bismaleimides improved the fracture toughness of the materials due to lower built-in cure stresses.
U.S. Pat. No. 4,568,733 issued on Feb. 4, 1986, relates to mixed aromatic bismaleimide/aromatic biscitraconimide resins which produce materials which have better handling, processing and thermal properties than materials with individual resins. These resins are thermally cured without a curing catalyst. However, the incorporation of the biscitraconimide into the bismaleimide generally produced a significant reduction in the elongation percent.
In EP 0407661 curable bisimide compositions are disclosed containing at least one biscitraconimide unit wherein the composition comprises an amount of an anionic curing catalyst to convert groups on the biscitraconimide units to alkylene bridges in the cured composition. In example 8 of this patent application a method is described for making a composition of biscitraconimide oligomers by radical polymerization. A mixture of n-hexamethylenebiscitraconimide, styrene, and dicumyl peroxide was polymerized and the composition thus obtained was used for the preparation of prepregs. It was, however, found that the viscosity of this composition was by far too low, so that it could not be used in commercial scale prepregging and laminating processes. Moreover, the reactivity of this prepolymer was low so that the gel time needed for the final curing process could not be reached.
Generally, the commercially used bismaleimide resins are all based on co-oligomers made from almost stoichiometric amounts of bismaleimides and co-monomers such as diamines, diallyl compounds, and require difficult processing conditions. In addition, the maleimide polymers are often brittle due to the high cross-link density obtained in the network polymers, whereas the maleimide oligomer of EP 0407661 has low flowing ability when applied onto fabric. The foregoing body of prior art reflects the need for bisimide resin systems which are easily processable, are made from almost solely a biscitraconimide monomer, and have lower cost.